1. Field of the Invention
The present invention relates generally to a method of generating a heat-plasma suited for use in providing a surface coating intended for electric insulation, thermal insulation, resistance to abrasion, resistance to corrosion, or the like, or for use in providing an optically functional film or a magnetically functional film. More particularly, the present invention relates to a method of generating a heat-plasma suited for use in providing a surface coating in a relatively wide area in a heat-plasma spray coating or a CVD (Chemical Vapor Deposition) coating.
The present invention also relates to an apparatus for effecting the above-described method.
2. Description of the Prior Art
Conventionally, a spray coating technique has been widely used for a long time as a method of providing a surface coating having a resistance to abrasion, or a surface coating for insulation purposes. This technique is broadly classified into a gas spray coating in which combustion gases are used as a fusing means, an electric spray coating in which electric energy is used as spray coating energy, or the like.
An arc spray coating or a plasma spray coating is generally employed as the electric spray coating. Recently, attention has been paid particularly to plasma spray coating, in consideration of the quality of the coating film.
FIG. 1 depicts a conventional plasma spray coating apparatus having a water-cooled cathode 1 and a water-cooled anode 2, wherein a DC arc 4 is generated between the cathode 1 and the anode 2 by a power source 3. A plasma generating gas 5 introduced into the apparatus is heated by the arc 4 and is jetted out of a nozzle 7 as a high-temperature plasma 6. Powder, as a spray coating material, is introduced along with carrier gas 8 into the plasma jet, in which the powder is heated, fused and accelerated so that the powder may be caused to collide against the surface of a substrate 9 at a high speed to provide a surface coating. Argon gas or nitrogen gas is generally employed as the plasma generating gas, and hydrogen gas, helium gas, or the like is frequently added thereto.
As shown in FIG. 1, the cathode 1 and the anode 2 are coaxially disposed in this kind of plasma spray coating torch. Although the area of a plasma outlet of the nozzle 7 depends upon the output of the apparatus, the maximum area thereof is approximately 0.2 cm.sup.2. Accordingly, when the surface coating is conducted on a large-sized substrate having a wide area, for example an electronic display, using the spray coating or the heat-plasma CVD, the area must be enlarged by extending the distance between the torch and the substrate 9. Alternatively, a region 12 to be coated must be gradually enlarged by moving one of a torch 11 and a substrate 10 relative to the other, as shown in FIG. 2.
When the distance between the torch and the substrate is extended, the collision speed of fused particles against the substrate becomes slow, thus causing a resultant film of coating to be porous and considerably uneven.
On the other hand, the method as shown in FIG. 2 is disadvantageous in that the film of coating tends to become uneven in thickness, particularly, in the direction shown by an arrow (Y), and a device for moving one of the torch 11 and the substrate 12 relative to the other is expensive. Furthermore, since much time is required for the surface coating, this method is not suitable for mass-production.